U.S. patent number 4,883,486 [Application Number 07/200,427] was granted by the patent office on 1989-11-28 for prosthetic ligament.
Invention is credited to Indu Kapadia, Kemal Schankereli.
United States Patent |
4,883,486 |
Kapadia , et al. |
November 28, 1989 |
Prosthetic ligament
Abstract
A prosthetic ligament has an outer sheath of permeable PTFE yarn
surrounding a core of synthetic filaments with stitching through
the sheath and core holding the two firmly together to act as a
single integral unit when placed in situ.
Inventors: |
Kapadia; Indu (Denville,
NJ), Schankereli; Kemal (Stillwater, MN) |
Family
ID: |
22741685 |
Appl.
No.: |
07/200,427 |
Filed: |
May 31, 1988 |
Current U.S.
Class: |
623/13.15 |
Current CPC
Class: |
A61F
2/08 (20130101); A61L 27/16 (20130101); A61L
27/16 (20130101); A61F 2/08 (20130101); C08L
27/18 (20130101); A61L 27/16 (20130101); C08L
27/18 (20130101); A61F 2250/0098 (20130101) |
Current International
Class: |
A61F
2/08 (20060101); A61L 27/16 (20060101); A61L
27/00 (20060101); A61F 2/00 (20060101); A61F
002/08 () |
Field of
Search: |
;623/13,1,66
;128/334R,335.5 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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1178441 |
|
Sep 1985 |
|
SU |
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8500511 |
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Feb 1985 |
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WO |
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Primary Examiner: Cannon; Alan W.
Attorney, Agent or Firm: Jacobson and Johnson
Claims
We claim:
1. A ligament prosthesis for implanting in situ in a body,
comprising:
an elongated tubular porous sheath made of PTFE yarn;
an unwoven non-braided core within said sheath, said core
comprising a plurality of linearly arranged high tenacity strands
of polyester running generally parallel to one another and to the
central axis of said sheath;
said core being anchored to said sheath with a stitching of
multifilamentous synthetic thread passing through said sheath and
said core.
2. The prosthetic ligament as in claim 1 wherein said sheath has a
permeability sufficient to permit tissue ingrowth into the sheath
when placed in situ.
3. The prosthetic ligament as in claim 1 wherein said sheath is
characterized by having a permeability in the range of about 20 to
200 ml/min/cm.sup.2 and a density of about 40-70 picks per
inch.
4. The prosthetic ligament as in claim 1 wherein said stitching is
a continuous zig-zag or wavy pattern along the length of the sheath
and in the range of about six to fifteen stitches per inch.
5. The prosthetic ligament as in claim 4 wherein the sheath is
generally elliptical in cross-section.
6. The prosthetic ligament as in claim 1 wherein said core
comprises in the range from about six to about seventy strands.
7. The prosthetic ligament as in claim 6 wherein said strands each
have a tenacity greater than about five grams per denier.
8. The prosthetic ligament as in claim 1 further including a
radiopaque monofilament in said core.
9. The prosthetic ligament as in claim 1 further including a
collagen coating on said core strands and on said sheath for
inducing tissue growth.
10. The prosthetic ligament as in claim 1, said ligament having two
ends and further including a guide tip attached to an end of said
ligament.
11. The prosthetic ligament as in claim 1,
said ligament having two ends and further including
a loop at an end of said ligament formed by looping the end back
onto the ligament and attaching it thereto.
12. The prosthetic ligament as in claim 1 wherein said sheath is
made of bleached 225 denier 30 filament yarn of about 40-70 picks
per inch.
13. The prosthetic ligament as in claim 1 wherein said core
comprises in the range of about 6 to 70 strands of yarn each in the
range of about 0.47 square mms. in cross-sectional area.
14. The prosthetic ligament as in claim 1 wherein said sheath yarns
are braided.
15. The prosthetic ligament as in claim 1 wherein said sheath yarns
are woven.
16. The prosthetic ligament as in claim 1 wherein said sheath yarns
are knitted.
17. The prosthetic ligament as in claim 1 wherein said stitching
comprises a series of separate cross-stitches spaced from one
another along the length of the sheath.
Description
FIELD OF THE INVENTION
This invention is directed toward providing a prosthetic ligament
utilizing synthetic materials.
DESCRIPTION OF THE PRIOR ART
Replacement of anatomical parts using prosthetic devices designed
to carry out similar functions frequently becomes the solution of
choice when other medical alternatives have been exhausted.
Numerous prosthetic devices are available as replacements for
almost all of the major anatomical functions. For example,
orthopedic surgery for replacement of diseased hips and knees with
prosthetic hip and knee joints has become quite common as effective
devices have been developed. However, a similar trend has not been
noted for ligaments, largely due to the fact that adequate
prosthetic devices have not yet been developed.
A conventional medical intervention for the repair of damaged
ligaments involves the surgical harvesting of tissue from one
portion of the body to be used for ligament repair elsewhere. One
example is the use of fascia lata (fascial strip) to repair damaged
ligaments. This is not satisfactory since extensive surgery is
involved, and the repair or replacement tissue does not always
function adequately in situ.
The medical community has prescribed certain characteristics for
prosthetic ligaments. While all the properties influencing the
ultimate success of a ligament prosthesis have not yet been
defined, the following are some of the most important desired
characteristics:
1. Adequate strength;
2. Resistance to elongation;
3. Fixation methods, the device should be easy to implant and
attach;
4. Biocompatability, as demonstrated by a minimum of inflammatory
responses;
5. Longevity, the device should last the lifetime of the
patient;
6. Tissue ingrowth, host tissue should be able to penetrate the
device to stabilize and ultimately enhance the device's physical
property;
7. Activity, the implanted device should allow early if not
immediate use of the limb;
8. Pliability;
9. Resistance to abrasion.
Various attempts have been made to design devices that meet the
aforementioned criteria. One such device, described in U.S. Pat.
No. 4,483,023 by Hoffman, et al., comprises a knitted polyester
sheath surrounding a core of polyester strands. One of the
deficiencies of the Hoffman device is that the longevity of the
device is likely to be severely limited by a slow degradation of
the polyester materials used to construct the sheath of the device.
Invivo experience utilizing similar polyester yarns for
cardiovascular devices has indicated that substantial losses of
strength may accompany the use of these lower tenacity polyester
yarns in implanted devices. Further, tissue ingrowth is likely to
be slowed by the knitted polyester sheath which ultimately slows
the healing process.
The device constructed in accordance with the teachings of the
Hoffman '023 patent contains a woven or braided core. While woven
core yarns may possess adequate strength, they intrinsically are
less pliable thus detracting from this important aspect of the
device. Alternately, braiding of the core yarn which might improve
the pliability of the device would lead to a construction
intrinsically lower in tensile strength as well as a device that
would be subject to early failure resulting from the cutting action
of the braided core yarns upon themselves when they were subjected
to stress loads.
U.S. Pat. No. 4,149,277 introduces another variant in the
construction of prosthetic ligaments and tendons in the form of
carbon coated polyester filaments in braided, woven or meshed
array. Carbon has long been known to be effective in improving the
biocompatability of biomedical devices. Carbon coated polymeric
devices constructed in accordance with the teachings of the '277
patent would expectedly exhibit better biocompatability than most
noncoated polymeric constructions. However, as a result of the
contact of yarns in braided, woven, or meshed constructions
ultimately a delamination of carbon from these points could occur,
thus reducing the biocompatability of the device. In addition, the
devices constructed in accordance with the teaching of the '277
patent would suffer from the same problems encountered for woven or
braided constructions described in the '023 patent.
U.S. Pat. No. 4,585,458 describes the use of chemically fixed
heterologous collagenous tissues instead of synthetic materials for
repair or replacement of ligaments or tendons. At present such
devices have not proven to be effective alternatives for ligament
repair since they typically exhibit premature failure.
SUMMARY OF THE INVENTION
An improved ligament prosthesis exhibiting greater pliability and
improved host tissue response is constructed with an outer sheath
of braided yarns of polytetrafluoroethylene (PTFE), commonly known
under the trademark TEFLON, and a lumen core composed of multiple
high tenacity yarns or strands of synthetic materials such as
polyester lying in linear array along or parallel to the
longitudinal axis of the sheath. Multifilamentous PTFE and
polyester yarns were first investigated for medical applications
during the 1950's. Since that time experiments and experiences with
these materials have established that they are suitable for
biomedical applications and have most, if not all, of the desired
characteristics when used in proper combination to produce
prosthetic ligaments.
To insure an integral unit that will uniformly respond to stresses,
the sheath and core are attached together by stitching the two
components together using a polyester or PTFE multifilamentous
yarn. To expedite healing, collagen materials as well as polylactic
acid may be coated onto the core yarns as well as the sheath
material. As a further feature, a loop may be provided for
attaching the device to a bone at one end by placing an anchoring
bone screw through the loop when the device is placed in situ.
Yet another feature of the invention is the inclusion of a
radiopaque filament such as barium sulfate (Ba So4) filled
polypropylene in the core to assist in x-ray viewing of the device
after it has been implanted in situ.
By placing the core strands or yarn side-by-side linearally along
or parallel to the longitudinal axis of the sheath, the strands are
prevented from crossing over one another thereby eliminating or
minimizing the possibility of abrasion which might otherwise weaken
or tear the core strands after implant. Since the core yarns are
not woven nor braided together, the device exhibits greater
pliability and thus constitutes a major improvement in regard to
surgical utility.
Stitching through the sheath and the core strands in a zig-zag or
wavy pattern, or, alternatively with an evenly spaced cross (x)
pattern, holds the two together so the device will operate in situ
as an integral unit and there will be relatively little movement
between the outer sheath and the core.
The sheath made of braided PTFE yarn is porous to provide a more
satisfactory controlled ingrowth of host tissue to stabilize the
device. The tissue ingrowth into the device constructed utilizing
multifilamentous PTFE yarns exhibits a more uniform, less
inflammatory tissue (host) response than that noted in polyester
devices. Also, PTFE is highly resistant to degradation by chemical
activity when in situ, it is highly inert, and because of its low
coefficient of friction, is less susceptible to damage by
abrasion.
A coating material such as polylactic acid or collagen facilitates
the healing and further enhances the biocompatability of the device
in situ while it may also act as a carrier for any medications such
as antibiotics, which might be administered.
As a further feature, the device may be provided with a removable
introducer tip for use in stringing the prosthetic ligament when
being placed in situ. Alternatively, the ends may be potted or
stiffened in a suitable material such as polyurethane or silicone
for this same purpose.
DESCRIPTION OF THE DRAWING
FIG. 1 is what might be considered a side view of a preferred
embodiment of a prosthetic device constructed according to the
teachings of the invention with a zig-zag or wavy continuous
stitching pattern;
FIG. 2 is correspondingly what might be considered a top or bottom
view of the embodiment shown in FIG. 1;
FIG. 3 is a somewhat enlarged cross-section view showing the
relationship of the core yarns to the sheath before stitching;
FIG. 4 is an illustration of an embodiment of the invention placed
in situ;
FIG. 5 illustrates an alternate stitching pattern;
FIG. 6 is a graph illustrating the tensile strength of the core
yarns used in an embodiment of the invention; and
FIG. 7 is a graph illustrating the tensile strength of an
embodiment of the invention.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
The prosthetic device, designated generally by reference numeral
10, has an outer tubular sheath 11 surrounding a core 17 of
elongated multifilamentous relatively high tenacity yarns or
strands 12 of a synthetic material such as polyester. Each of the
strands or yarns 12 is made up of a multitude of fibres or
filaments 23. The individual core strands 12 are arranged to rest
side by side linearally along and parallel to one another and
parallel to the longitudinal axis of the sheath lumen. The fabric
of sheath 11 is made of braided multifilamentous strands of PTFE as
compared to prior art devices, such as in the Hoffman U.S. Pat. No.
4,483,023, in which the sheath fabric is made from knitted strands
of polyester. As mentioned earlier, both fabrics have some degree
of permeability which permits tissue ingrowth but it has been found
that the that the fabric made from braided PTFE produces a more
satisfactory controlled tissue ingrowth apparently due to a
combination of the braiding and the characteristics of the PTFE.
Also, knitted polyester fabric is likely to deteriorate more
rapidly than knitted or braided PTFE when the ligament is placed in
situ where it is subjected to the continuous motions and stresses
encountered by the prosthetic ligament during normal use.
Core 17 is secured to sheath 11 by stitching 13 which passes
through sheath 11 and core 17 in a zig-zag or wavy pattern along
the entire length of the device. Stitching 13 randomly passes over,
under and between different strands 12 in core 17 as it passes back
and forth between sides of the sheath so that its overall effect,
over the entire length of the device, is to firmly hold core
strands 12 within sheath 11 so that the core and sheath generally
act together as a single integral unit with a minimum amount of
relative sliding motion between them. Preferably, stitching 13 is a
single continuous zig-zag or wavy stitching extending over the
entire length of the sheath, as illustrated in FIG. 1.
Alternatively, the stitching may be a series of separate transverse
or cross-stitches 21 spaced from one another along the entire
length of the sheath as illustrated in FIG. 5, or may take other
forms or patterns. The sheath may be closed off with a transverse
stitch 20 near each end. One end of the device may be provided with
a loop 16 which can be used to anchor one end of the prosthetic
device when it is placed in situ. Loop 16 is formed merely by
looping one end of the sheathed core back on itself and attaching
the end, preferably by secure stitching, utilizing a polyester or
PTFE thread.
Core 17 comprises in the range from about six to about seventy
individual lengths or ends or strands of a relatively high tenacity
polyester yarn such as, for example, 1100 denier, 192 filament
Dacron T-73. Dacron T-73 has a tenacity of about nine grams per
denier. Experience has shown that polyester having a tenacity
greater than about five grams per denier is satisfactory.
Preferably the packing density should be about sixty ends of yarn
in a sheath having an outer diameter of about six mm. As an added
feature, if it is desired to provide radiopacity, a radiopaque
monofilament 19 may be included in the core. Typically a suitable
monofilament of this nature will range from about five to fifteen
mils in diameter and contain in the range from about 8% to about
20% barium sulfate (Ba So4) or other suitable material.
Typically, the fabric of sheath 11 is made from a suitable PTFE
yarn, for example, a bleached 225 denier, 30 filament yarn
utilizing about 40-70 picks per inch and has a water permeability
ranging from about 20 to 200 ml/min/cm.sup.2 when measured using a
Wesolowski permeability tester @120 mm. Hg pressure as described in
4.3.1.2 of the American National standard (7-7-1986) for vascular
prosthesis developed by A.A.M.I. The sheath fabric may be
texturized, plyed or twisted to achieve surface characteristics
more advantageous to tissue ingrowth. Because of the permeability,
when the sheath is placed in situ host tissue ingrowth is
facilitate to and through the sheath and to the core. Experience
has shown that preferably the permeability should be in the range
mentioned above but theoretically, there is no upper limit to the
permeability provided the sheath can still perform its function of
securely holding the core in place and withstand the forces
normally encountered over extended periods of time after the
prosthetic device is placed in situ. In one embodiment core 17 is
integrally attached to sheath 11 by continuous zig-zag or wavy
stitching 13 which preferably is in the range of about 6-15
stitches per inch along the long axis of the device through sheath
11 utilizing a multifilamentous synthetic thread such as PTFE or
polyester. Initially, sheath 11 is cylindrical (see FIG. 3) but
after the core has been inserted and zig-zag stitching 13 added, it
takes on an elliptical shape. In this shape and form the device is
flexible enough so that when being placed in situ, it can be
inserted into and pass through any openings and can be turned and
twisted as necessary without losing any of its characteristics as a
suitable synthetic prosthetic ligament. Alternatively, sheath 11
may be attached to core 17 utilizing evenly spaced cross-stitches
21 as shown in FIG. 5. In this manner the cross section of the
device will be elliptical or flattened at the stitches but will be
generally circular between stitches, thus making it somewhat more
flexible.
To assist in placing the device in situ, it may be provided with an
introducer tip 18 at its distal end. Tip 18 is preferably a sleeve
of heat shrinkable polyolefin material which is slipped over one
end of the device and heated until it shrinks to firmly grasp onto
the outer sheath. It can then be used as an aid by the operating
physician to guide the prosthetic ligament into place and when the
ligament is in place tip 18 is removed and discarded and then the
end of the ligament is attached by the surgeon. Alternatively, an
introducer tip may be provided at both ends.
As mentioned earlier, core strands 12 and sheath 11 may be coated
to enhance stabilization of the implanted device by tissue
ingrowth. Coating materials composed of insoluble, fibrillar
collagen as well as soluble collagen may be used. As one example,
an insoluble collagen coating matrix is prepared by adding two
grams of collagen (for example a type sold under the brand name
Secol LH4320) and five grams glycerol or sorbitol to 93 ml. H.sub.2
O to form a 2% collagen slurry. The resulting slurry is heated
.sub.at about 100.degree. C. for a period of about twenty-five
minutes to denature the collagen. When the slurry has cooled, it is
introduced onto the core and mechanically massaged to insure
complete coverage of the core yarns. Similarly, the PTFE sheath may
be coated with the collagen slurry and is also mechanically
massaged so that the slurry works its way into the interstices of
the sheath in such a manner that the device is completely coated
with collagen. The entire device is carefully placed into a 2%
formalin bath for a period of about twenty-four hours to partially
stabilize the collagen matrix. Alternately, the coated device may
be placed on a manifold and vapor phase cross-linked using poly
oxymethylene fumes for a period of several hours. Finally, the
device is dried in a drying oven for about twenty-four hours at
about 50.degree. C. Subsequently the sheath and core are stitched
together as described earlier. Other combinations of collagen,
glycerol and H.sub.2 O can be used and the same benefits
derived.
Devices constructed in accordance with the teachings of this
invention, with and without collagen coating, have been implanted
subcutaneously in rats and qualitatively compared to polyester
sheathed devices. After thirty days, the devices were explanted and
histologically characterized. Tissue ingrowths were noted in both
cases. The granular response around the PTFE sheath appeared
extremely inert and did not exhibit the inflammatory responses
typically noted for polyester sheathed prosthesis. The tissue
ingrowth appeared to be more organized in the PTFE braided sheath
with the sheath and core strands coated with collagen.
For reference, the packing density of the prosthetic ligament
described herein can be generally considered to be the ratio of the
number of core strands or yarns to the cross-sectional area of the
sheath lumen prior to stitching. While to a degree the packing
density may be a matter of choice, in general it is preferred that
the packing density be such that the core strands or yarns
substantially fill the area of the sheath lumen. Typically, for
example, a Dupont Dacron T-73, 1100 denier, 192 filament occupies
about 0.47 square mms. of cross-sectional area so if a sheath
having a lumen diameter of approximately six mms. were utilized
then the core should comprise about 60 ends. One of ordinary skill
in the art can readily determine the packing density suitable for
any size ligament knowing the size of the sheath and the core
filaments. The packing density should be chosen such that the
ligament has the required tensile strength for the specific
application and that the stitching will cause the core to pack
tightly so that the core filaments are held firmly in place and the
core and sheath act as a single integral unit, minimizing the
possibility of any sliding motion between the core filaments and
the sheath.
FIG. 4 illustrates one manner in which an embodiment of the
invention may be placed in situ. A pin 22 may be inserted into the
femur to anchor loop 16 and the prosthetic ligament 10 is then
wrapped part way around the lower end of the femur and between the
femur and tibia and under the patella and attached to the tibia in
some convenient fashion by the operating surgeon.
FIG. 6 is a reproduction of an actual stress-strain curve of T-73,
1100/192 Dupont polyester yarn used in the core of an embodiment
constructed according to the teachings of this invention. It
illustrates that the core yarn will elongate up to about 10% of its
original length before it will break at an applied stress force of
about 23 pounds.
FIG. 7 is a reproduction of an actual stress-strain curve of a
prosthetic ligament constructed according to the teachings of the
invention having a thirty end core of the yarn tested in FIG. 6.
FIG. 7 illustrates that the ligament will elongate up to about 12%
of its original length before it will break at an applied stress
force of about 585 pounds.
* * * * *